This invention pertains to the field of separating and recovering at least one gas component from a feedstream by an adsorption process. More particularly, the present invention relates to a pressure swing adsorption (PSA) process for separating a hydrocarbon feed gas comprising propylene and propane into a fraction comprising predominantly propylene and a fraction comprising propane.
Light olefins serve as the building blocks for the production of numerous chemicals. Light olefins have traditionally been produced through the process of steam or catalytic cracking. Propylene, a light olefin consisting of three carbon atoms wherein two of the carbon atoms are joined by a double bond, has a great number of commercial applications, particularly in the manufacture of polypropylene, isopropyl alcohol, propylene oxide, cumene, synthetic glycerol, isoprene, and oxo alcohols. When propylene is produced in the presence of hydrogen, it is often accompanied by the formation of propane. Propane is a paraffin, a saturated hydrocarbon which is used as a component of household fuel, as an extractant, a refrigerant, or an aerosol propellant. Generally, it is required to separate propane from propylene before the propylene can be used to produce more valuable products. However, the boiling points of propane and propylene are very close to one another and separating propane from propylene has traditionally required an energy-intensive fractionation process known as superfractionation. Superfractionation generally refers to fractional distillation of compounds having the same number of carbon atoms per molecule.
The reaction product produced by a conversion process such as fluid catalytic cracking, pyrolysis of naphtha, and conversion of methanol to olefins is a light gas stream containing lighter components (e.g. hydrogen, nitrogen, etc.) methane, ethane and a substantial quantity of hydrocarbons of higher molecular weight, for example, propane, butane, pentane, and often their unsaturated analogs. Separation of these components to recover propylene requires a complex energy-intensive scheme, thus creating a need for more efficient separation processes which yield higher recovery levels of propylene. A typical ethylene separation section of an ethylene plant containing both cryogenic and fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti entitled, xe2x80x9cBetter Ethylene Separation Unit,xe2x80x9d that appeared in Hydrocarbon Processing, November 1988, pages 57-61 and is herein incorporated by reference. In the production of propylene, a by-product of the conversion process, a reactor effluent stream recovered from the conversion process is passed to a complex series of separation stages involving a combination of compression and fractionation steps to recover a C3 hydrocarbon stream. Conventionally, the C3 hydrocarbon stream is passed to a superfractionator known as a C3 splitter to perform the separation of propylene from a feedstream consisting essentially of propylene and propane to produce a high purity propylene stream and a propane-containing stream. The C3 splitter, or superfractionator, makes the final separation between propylene and propane. In this separation, propylene is removed as an overhead stream, and the propane stream is removed as the C3 splitter bottom stream. The higher the purity of the propylene desired, the more energy intensive is the degree of superfractionation in the C3 splitter. Typically, the C3 splitter or propane/propylene splitter requires so many theoretical separation stages, or trays, that the column is constructed as two separate towers: a rectifier and a stripper. Lighter components such as hydrogen and methane which may be present in the C3 hydrocarbon stream further complicate the separation.
PSA provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product which is separated from the less strongly adsorbable gas. In PSA, a multi-component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more co-current depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a counter-current depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas counter-currently to the direction of the feedstream. Finally, the adsorption zone is purged and repressurized. The combined gas stream produced during the counter-current depressurization step and the purge step is typically referred to as the tail gas stream. The final stage of repressurization is typically performed by introducing a slipstream of product gas comprising the lightest gas component produced during the adsorption step. This final stage of repressurization is often referred to as product repressurization. In multi-zone systems, there are typically additional steps and those noted above may be done in stages. U.S. Pat. No. 3,176,444 issued to Kiyonaga, U.S. Pat. No. 3,986,849 issued to Fuderer et al., and U.S. Pat. Nos. 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic PSA systems employing both co-current and counter-current depressurization and the disclosures of these patents are incorporated by reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factors. Molecular sieves such as the microporous crystalline zeolite and non-zeolitic catalysts, particularly aluminophosphates (AWPO) and silicoaluminophosphates (SAPO), are known to promote reactions such as the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. No. 4,310,440 (Wilson et al.); U.S. Pat. No. 4,567,029 (Wilson et al.); U.S. Pat. No. 5,095,163 (Barger); U.S. Pat. No. 5,191,141 (Barger); U.S. Pat. No. 5,126,308 (Barger); U.S. Pat. No. 4973792 (Lewis); and U.S. Pat. No. 4,861,938 (Lewis). The above U.S. patents are hereby incorporated by reference.
U.S. Pat. Nos. 5,744,687 and 5,365,011 disclose a process for the integration of a PSA zone containing an adsorbent selective for the adsorption of ethylene and propylene from a catalytic cracking process at an adsorption temperature above 50xc2x0 C. to about 250xc2x0 C. The adsorbent is selected from the group consisting of zeolite 4A, zeolite 5A, zeolite 13xc3x97, and mixtures thereof. The adsorbed ethylene and propylene is desorbed from the adsorbent by reducing the pressure or raising the temperature, or by reducing the pressure and raising the temperature.
A paper entitled, xe2x80x9cOlefin-Paraffin Separations by Adsorption: Equilibrium Separation by xcfx80-Complexation vs. Kinetic Separationxe2x80x9d, by S. U. Rege, J. Padin, and R. T. Yang and published in the AIChE Journal, volume 44, 1998, at page 799 and herein incorporated by reference, compares the performance of a PSA process using 4A zeolite, carbon molecular sieve, and a sorbent based on a silica substrate over which silver nitrate has been dispersed.
Recovering propylene from cracking and oxygenate conversion processes is an expensive and complex process involving extensive compression and superfractionation to separate ethylene from the C3 hydrocarbons and finally superfractionation to separate the propylene from the propane. Processes are sought which enable the concentration and recovery of the propylene from cracking and oxygenate conversion effluent without expensive superfractionation steps.
It is an objective of the present invention to provide a separation process for the production of high purity propylene which does not require superfractionation.
It was discovered that the non-zeolitic molecular sieve AlPO-14 at a temperature above 120xc2x0 C. and below a temperature of incipient reaction can be employed as an adsorbent in a PSA process to replace the superfractionation step in cracking and oxygenate conversion schemes to produce a high purity propylene product. AlPO-14 has been generally considered as a catalyst for hydrocarbon conversion reactions. Its narrow, small pore structure was discovered to be useful for adsorbing propylene while essentially excluding the co-adsorption of propane when the adsorption temperature was above the adsorption temperature of 120xc2x0 C. Furthermore, the working capacity of the AlPO-14 did not become significantly lessened by successive adsorption/desorption cycles. The use of the PSA process wherein the selective adsorbent comprises AlPO-14 and the adsorption temperature is 120xc2x0 C. provides a process for the production of a high purity propylene product without using conventional superfractionation methods. The resulting process developed for propylene separation will result in a propylene/propane separation zone of lower capital cost and loser operating cost than conventional C3 splitter based schemes.
In one embodiment, the present invention is a process for the production of a high purity propylene stream from a mixture comprising propane and propylene. The process comprises contacting the mixture with a selective adsorbent comprising AlPO-14 at a temperature between 120xc2x0 C. and below a temperature of incipient reaction at an adsorption pressure in an adsorption step to provide an adsorption effluent stream comprising propane, and in desorption step at a desorption pressure lower than the adsorption pressure desorbing propylene to provide a propylene product stream.
In another embodiment, the present invention is a PSA process comprising a series of steps. A feed stream comprising propylene and propane and a recycle stream comprising propylene are passed at an adsorption temperature between 120xc2x0 C. and below a temperature of incipient reaction and an adsorption pressure to a first adsorption bed in an adsorption zone comprising at least two adsorption beds and an adsorption effluent comprising propane is withdrawn. Each of the adsorption beds contains an AlPO-14 adsorbent for the selective adsorption of propylene. The passing of the feed stream and recycle stream to the first adsorption bed is terminated prior to breakthrough of propylene. The first adsorption bed is co-currently purged in a co-purge step with a purge gas stream at a purge pressure essentially equal to the adsorption pressure and a purge effluent stream is withdrawn from the first adsorption bed. The co-purge step is terminated and the first adsorption bed is desorbed in a counter-current blowdown step to provide a high purity product stream comprising propylene. At least a portion of the high purity product stream is compressed to provide the purge gas stream and the purge effluent stream is recycled to the adsorption step to provide the recycle stream. The first adsorption bed is co-currently repressurized with the feed stream. The above process steps of adsorption, co-current purge, counter-current blowdown, and repressurization are repeated to provide a continuous process.